Our ears pick up all the sounds around us and then translate this information into a form the brain can understand. One of the most remarkable things about this process is that it is completely mechanical. Other senses, smell, taste and vision, involve chemical reactions, but the hearing system is based solely on physical movement.
How We Hear
Hearing depends on a series of events that change sound waves in the air into electrical signals. Our auditory nerve then carries these signals to the brain through a complex series of steps.
- Sound waves enter the outer ear and travel through a narrow passageway called the ear canal, which leads to the eardrum.
The eardrum vibrates from the incoming sound waves and sends these vibrations to three tiny bones in the middle ear. These bones are called the malleus, incus, and stapes.
- The bones in the middle ear amplify, or increase, the sound vibrations and send them to the inner ear—also called the cochlea—which is shaped like a snail and is filled with fluid. An elastic membrane runs from the beginning to the end of the cochlea, splitting it into an upper and lower part. This membrane is called the "basilar" membrane because it serves as the base, or ground floor, on which key hearing structures sit.
The sound vibrations cause the fluid inside the cochlea to ripple, and a traveling wave forms along the basilar membrane. Hair cells—sensory cells sitting on top of the membrane—"ride the wave."
- As the hair cells move up and down, their bristly structures bump up against an overlying membrane and tilt to one side. This tilting action causes pore-like channels, which are on the surface of the bristles, to open up. When that happens, certain chemicals rush in, creating an electrical signal.
The auditory nerve carries this electrical signal to the brain, which translates it into a "sound" that we recognize and understand.
- Hair cells near the base of the cochlea detect higher-pitched sounds, such as a cell phone ringing. Those nearer the apex, or centermost point, detect lower-pitched sounds, such as a large dog barking.
Why Does Hearing Loss Happen?
Hearing loss happens for many reasons. In most cases, hearing loss results from damage to the inner ear. Aging and prolonged exposure to loud noise may cause wear and tear on the hairs or nerve cells in the cochlea that send sound signals to the brain. When these hairs or nerve cells are damaged or missing, electrical signals aren't transmitted as efficiently, and hearing loss occurs.
Some people lose their hearing slowly as they age. This condition is called presbycusis. Although prescycusis is irreversible, there are many products to help a person hear better. With assistive technology, people with hearing loss can lead normal lives and communicate more effectively.
The National Institute on Deafness and Other Communication Disorders (NIDCD) is one of the Institutes that comprise the National Institutes of Health (NIH) and focuses on the prevention, detection, diagnosis and treatment of hearing loss and deafness.